linux-kernel - Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

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Message-ID: <5045E387.4030103@redhat.com>
Date:	Tue, 04 Sep 2012 13:18:31 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org, rusty@...tcorp.com.au, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

Il 04/09/2012 13:09, Michael S. Tsirkin ha scritto:
>> >     queuecommand on CPU #0         queuecommand #2 on CPU #1
>> >   --------------------------------------------------------------
>> >     atomic_inc_return(...) == 1
>> >                                    atomic_inc_return(...) == 2
>> >                                    virtscsi_queuecommand to queue #1
>> >     tgt->req_vq = queue #0
>> >     virtscsi_queuecommand to queue #0
>> > 
>> > then two requests are issued to different queues without a quiescent
>> > point in the middle.
> What happens then? Does this break correctness?

Yes, requests to the same target should be processed in FIFO order, or
you have things like a flush issued before the write it was supposed to
flush.  This is why I can only change the queue when there is no request
pending.

Paolo
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